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EXTRACTIONEXTRACTION
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Introduction
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Extraction: refers to an operation in which one or
more component of a liquid or a solid aretransferred to another liquid
When liquid-liquid extraction is used, at least oneof the liquid components must be sucientlyinsoluble so that two liquid phases are formed
For example: the separation of certaincomponents of petroleum-based oils by the use
of acetone !f acetone is added to the oil, some
of the components of the oil dissol"e in theacetone solution, and the remainder of the oilforms an immiscible #non-mixin$% layer with theacetone solution &hese two layers can beseparated
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&he transfer of material from one phase toanother ta'es place by difusion
(i)usion may here be de*ned as thespontaneous intermin$lin$ of the particlesformin$ a solution
For example: when ammonia "apor isabsorbed in water, the ammonia molecules orparticles must di)use throu$h the $as to the
surface of the liquid water &hese ammoniamolecules must then enter the liquid solutionand di)use away from the water surface intothe main body of the liquid
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Liquid-liquid Extraction
iquid-iquid extraction #E% is a masstransfer operation in which a liquid solution#the feed% is contacted with an immiscible ornearly immiscible liquid #sol"ent% that exhibitspreferential anity or selecti"ity towards oneor more of the components in the feed
&wo streams result from this contact: theextract, which is the sol"ent rich solutioncontainin$ the desired extracted solute, andthe ranate, the residual feed solutioncontainin$ little solute
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E is often used to separate two mutually solubleliquids
/ third liquid #solvent%, ha"in$ a preferentialsolubility for one of the components of the mixture,must be added to the solution, and a separate liquidphase must be formed
&he sol"ent ta'es up part of the components of theori$inal solution and forms an immiscible layer withthe remainin$ solution
&he sol"ent layer is called extract
&he remainder of the ori$inal solution #may containsome of the sol"ent% is termed as ranate
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Factors to be considered when optimiin$ the
desi$n and operation of the extractionprocesses:
ol"ent selection
3peratin$ 4onditions
5ode of 3peration Extractor &ype
(esi$n 4riteria
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Solvent selection
ol"ents di)er in their extraction capabilitiesdependin$on theirown and the solute7schemical structure 8identify the desired unctional rou!"s#in the sol"entfor any $i"en solute
&he desired properties of sol"ents are a $i$distri%ution coecient& ood selectivit' towardssolute and little or no miscibility with feed solution/lso, the sol"ent should be easily reco"erable forrecycle
3ther factors a)ectin$ sol"ent selection are boilin$point, density, interfacial tension, "iscosity,corrosi"eness, 9ammability, toxicity, stability,compatibility with product, a"ailability and cost
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O!eratin conditions
(ependin$ on the nature of the extraction process, thetemperature, p; and residence time could ha"e an e)ecton the yield and selecti"ity
3ther parameters to be considered are selecti"ity, mutual
solubility, precipitation of solids and "apor pressure
&he p; becomes si$ni*cant in metal and bio-extractions
!n bio-extractions #e$,
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(ode o o!eration
Extractors can be operated in crosscurrent or counter-current mode
Cross-current
5ainly used in low capacity multi-product batchprocesses
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Extractor t'!e
4ommercially important extractors can beclassi*ed into the followin$ broad cate$ories:
(ixer-settlers
usually a series of static or a$itated mixersinterspersed with settlin$ sta$es &hese aremostly used in the metal industry whereintense mixin$ and hi$h residence time isrequired by the reacti"e extraction processes
!n batch mode of operation, these mixer-settlers could be simple batch "essels wherefeed and sol"ent are mixed and settled &hisoperation is repeated with fresh sol"ent
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MIXER SETTLER
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Centriual extractors
are hi$h-speed rotary machines that o)er
ad"anta$es of "ery low residence time &he number of sta$es in a centrifu$al de"ice
is usually limited to one, but currentlyde"ices with multiple numbers of sta$es arecommon
&hese extractors are mainly used inpharmaceutical industry
Colu)n contactors "static#
Colu)n contactors "aitated#
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KARRColumn
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SCHEIBELColumn
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1=
rotating-disk contacter
a. agitator; b. static disk
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Factors a)ectin$ the selection of extractors
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*esin criteria
&he basic function of extraction equipment is to mix twophases, form and maintain droplets of dispersed phaseand subsequently separate the phases
(ixin
&he amount of mixin$ required is determined byphysicalproperties such as "iscosity, interfacialtension and density di)erences between the twophases
!n a$itated batch extractors, the a$itator desi$n is
often optimied for reaction and heat transfer, notextraction
tatic extraction columns rely completely on thepac'in$@internals and 9uid 9ow "elocities past theinternals to create turbulence and droplets
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Settlin
&he settlin$ characteristics depend on the 9uidproperties #density di)erence, interfacial tension, and
continuous phase "iscosity% and the amount of mixin$ ettlin$ in a$itated batch "essels is carried out by
stoppin$ the a$itator
!n continuous columns, a settlin$ section is pro"idedeither as a part of theextractor or as a separate
piece of equipment downstream of the extractor
Selection o continuous and dis!ersed !$ases
!n column extractors, the phase with the lower"iscosity #lower 9ow resistance% is $enerally chosen
as the continuous phase &he phase with the hi$her 9ow rate can be dispersed
to create more interfacial area and turbulence
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Calculations or Extraction
4onsider a dilute solution of acetaldehyde intoluene, where part of the acetaldehyde is to beextracted usin$ water as the extractin$ sol"ent
&oluene and water are practically immiscible in
mixtures containin$ up to 10 wt A acetaldehydeWhen water is added to the solutions, some
acetaldehyde dissol"e in water and two liquidlayers are formed
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WA + T
A + W
A + T
Extract
Raffinate
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et
BoC '$s of acetaldehyde per '$ of toluene in the
initial solution
oC '$s of water #the extractin$ sol"ent% added
;oC '$s of toluene in the initial solution
D1C the equilibrium concentrations of acetaldehyde in
the extract in '$s of acetaldehyde per '$ of water
B1C the equilibrium concentrations of acetaldehyde
in the ranate in '$s of acetaldehyde per '$ oftoluene
/t equilibrium, the o"erall material balance on theacetaldehyde
Bo;oC ;oB1 oD1
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Exa)!le+
/ solution of acetaldehyde and toluene
containin$ 11 '$ of acetaldehyde and 1?? '$ oftoluene is to be extracted by =? '$ of water&he extraction is to be performed at 1 o4,where the equilibrium relationship between theacetaldehyde concentration in the extract and
ranate may be expressed as D1C 22B1 ;owmany 'ilo$rams of acetaldehyde will beremo"ed by the water
#/nswer: ?2 '$%
2.
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